Micro-pipetting of samples of fluid such as blood by use of small volume capillary tubes is a highly developed and advanced state of art. It is conventional in the known system to provide a shield for the pipette when it is not in use as a protective device. The shield is removably positioned on the pipette assembly so that it can be removed from the pipette when the pipette is introduced to the sample producing source for pipetting activity. Throughout the years, the shield has been used for various other purposes. For example, the shield is often used as a puncturing device since it has a closed protective end and forms a cap for the pipette. The closed end can be used to puncture diaphragms on reservoirs containing diluents and other types of medicaments to be used with the sample collected in the pipette.
Micro-pipettes with protective shields are known in many diverse fields. One particular area of common use is in the medical field where small samples of fluid such as precise micro-quantities of blood are collected and tested. Naturally other pipetting fields also require the use of a protective shield to guard the pipette when it is not being used. An example of a prior art patent in this area relating to general pipetting procedure and where a protective shield is employed is Roach U.S. Pat. No. 3,494,201 issued on Feb. 10, 1970. In contrast, examples of the type of pipette assembly under consideration which pertain to the medical profession are disclosed in U.S. Pat. Nos. 2,965,255 to Gerarde on Dec. 20, 1960; 3,433,712 to Gerarde on Mar. 18, 1969; 3,518,804 to Gerarde on July 7, 1970; and 3,779,083 to Ayres Et Al on Dec. 18, 1973. These references disclose the general pipetting concept and various types of known protective shields used with the pipette.
During the procedure of obtaining samples of fluid and diluting them with reagents contained within a resilient and compressible reservoir, spillage of the diluted sample can easily occur. Conventional pipette assemblies typicaly include three basic parts: a holder, a pipette mounted on one end of the holder, and a small overflow chamber on the other end of the holder. The holder is hollow so as to establish fluid communication between the pipette and overflow chamber.
In operation, a blood sample is first taken by touching the tip of the pipette to a supply of blood, commonly the patient's finger. The pipette fills by capillary action. A resilient reservoir containing diluent and reagents is squeezed slightly, and the pipette is inserted within the reservoir. When pressure is released on the reservoir, negative pressure draws the blood sample into the diluent. The reservoir is then squeezed gently several times to rinse the capillary bore, forcing diluent into, but not out of, the overflow chamber. Pressure is released each time to return the mixture to the diluent.
It is during this last step that great care must be taken in not squeezing the reservoir too hard. If overflow occurs, not only is there a loss of specimen sample, but there is also a risk of contaminating the fingers with the diluent solution. The solution often contains toxic compounds such as hydrazoic acid, and it is undesirable to have such chemicals contact the skin.